1. Field of the Invention
The present invention relates to a high-density optical disk with a bonded structure in which two substrates, each of which form an information signal layer, are bonded together.
2. Description of the Prior Art
Optical disks such as compact disks (hereafter referred to as "CDs"), which have existed for some time, have come into wide usage due to the amount of information they are able to store as well as their ease of handling. These CDs are formed by one substrate and, therefore, have a so-called single substrate structure.
Specifically, the thickness of the CD is approximately 1.2 mm (.+-.0.1 mm), and the distance from the light incidence surface, into which the playback laser beam enters to the information recording layer, is 1.1 to 1.3 mm.
Information which is in a format which is compatible with CDs is recorded onto this information recording layer, and this information is formed at a minimum pit length of 0.9 micron meter (.mu.m) and a track pitch of 1.6 micron meter (.mu.m) or less.
On the other hand, as shown in FIG. 1, there is a type of optical disk 100 with a bonded structure which forms two information recording layers by using a transparent bonding agent to bond a substrate a1, which forms the first information recording layer a, and a substrate b1, which forms the second information recording layer b, so that the information recording layers, a and b, are facing each other.
As shown in FIG. 1, this type of optical disk 100 is such that a playback laser beam is irradiated from both sides of the disk to read the information.
Compared to the single substrate structure, since this bonded structure has little disk warpage, and it can store twice as much information, it is increasingly used to enlarge the storage capacity of high-density optical disks.
High-density optical disks are optical disks which record information at a higher density than CDs by recording smaller recording pits at a more compressed track pitch.
With a high-density optical disk with the aforementioned bonded structure (hereafter this may be simply described as "high-density optical disk"), it is conceivable that the various types of optical aberrations caused by the thickness of the substrate can be reduced by making the bonded substrates thinner than that of the CD. For example, it is conceivable that a structure can be made in which the thicknesses, Da and Db, of the substrates, a1 and b1 shown in FIG. 1, are set to 0.6 mm, and by bonding these substrates together, a1 and b1, the thickness D of the disk is set to 1.2 mm.
Because the high-density optical disk 100 with the aforementioned bonded structure is made under the assumption that it will be played back on a player (hereafter described as a "high-density playback player") which is specifically for this high-density optical disk, it cannot be played back on a CD player. This is because the information recording layer is formed at the middle of the direction of the thickness of the disk. That is, because this high-density optical disk has a structure which has a first information recording layer a and a second information recording layer b at the middle of the direction of the thickness of the disk with the thicknesses, Da and Db, of substrates a1 and b1 as half of the thickness of the CD, the optical length of the optical pickup differs from that of the CD, and, for the time being, playback on a CD player is not possible even if CD information signals are recorded on one of the information recording layers.
In this way, the conventional high-density optical disk with a bonded structure has a disadvantage in that it cannot be played back on existing CD players.
On the other hand, a known type of disk with a layered structure which is different from the aforementioned high-density disk 100 with a bonded structure is a high-density disk which, by means of a transparent layer, stacks multiple reflective layers (information recording layers) in which the reflected light ranges mutually differ (for example, the "Optical Information Recording Medium and Its Method of Manufacture" described in Laid-Open Patent Publication No. HEI 2-223030).
However, there is no description which relates to also making it possible to play back this high-density disk with a layered structure on an existing disk player by making it compatible with existing disks; only an optical disk which improves the recording density through a layered structure is presented.
That is, an area that is not touched upon at all is that, as a result of the high-density disk being such that (1) the thickness of the disk is made the same as the thickness of existing CD disks, (2) the diameter of the disk is made the same as the diameter of existing CD disks, and (3), CD information signals are recorded on one of the multiple reflective layers (information recording layers), and the distance from the light incidence surface of the high-density disk to the information recording layer where the CD information signals are recorded is made the same as an existing CD's distance from the light incidence surface to the information recording layer; from the standpoint of the outer appearance and playback characteristics of the disk, even in the case where this high-density disk is loaded into an existing CD player, it is recognized by the CD player just as if an existing CD were loaded, and it is therefore possible to play back this high-density disk in an existing CD player.